Surface coatings originated in the Stone Age. Paint was first used by early Egyptians who dispersed pigment in a binder such as egg white. Today, paint is still basically a uniform dispersion of a binder or vehicle and a pigment. The vehicle is usually made up of a film-forming component, such as a resin; the vehicle is thinned with a solvent. With the exception of a minor percentage of powdered and curable solid surface coatings, currently the coating and finishing industry is predominantly based upon solvent-containing coatings.
The coating and finishing industry has, however, focused with great intensity upon its operations and their effect upon man's environment. Present coating techniques tend to create odors, smog, health and safety hazards. Legislation towards reducing such hazards at all levels of coatings manufacture and use is well advanced and enforced. However, compliance is not resulting in substantial changes in types of coatings used, rather, coatings are almost exclusively based on the solvent systems. Perhaps the most serious concern of the industry today, from a standpoint of both raw materials and environmental control, is the solvent components of the paint. Related concerns are the high price of energy, labor costs and capital in converting paints and liquid coatings into useful films.
The problems of the industry are illustrated by the commonly employed processes of liquid spray-coating, electrostatic liquid spray-coating and electrostatic powder-coating. In the spray-coating application of a resinous material, it is usual to dissolve the resinous material in an organic solvent to provide a suitable viscosity for spraying. Such methods of spraying solvent mixtures of film-forming resinous materials require significant amounts of solvent and loose solvent in handling, coating and finishing useful articles. Electrostatic liquid spray-coating techniques have been employed for coating normally liquid materials, i.e., paints or solvent coatings which have been atomized by air, airless or centrifugal atomization techniques. With respect to each of the spray-coating techniques, it is therefore common practice to dissolve a film-forming solid in an organic solvent to allow the composition to be handled, atomized and deposited upon the article to be finished. In fact, in known liquid spraying techniques, it is usually essential to use a solvent for the resinous coating composition in order to obtain a satisfactorily sprayed coating. During handling, atomization or deposition of solvent coating compositions, solvents will escape, and if not effectively trapped, the solvents will become air contamination. Even after a solvent coating is spread or applied to the article, solvents leave or escape from the coating film by evaporation and these too become contaminants of the surrounding atmosphere. Furthermore, since most solvents react with oxidants, they contribute to the pollution problems not only by their toxicity and unpleasant odors but also by creating smog. Organic solvents are further released during oven baking operations on coatings and are carried from the baking oven to the atmosphere in the form of exhaust pollutions. In an attempt to overcome the pollution problems associated with solvent spray-coating techniques, sophisticated recovery and after burner systems are employed to trap or burn solvent effluents. The cost of installation and operation of such systems and incinerators to dispose of the waste solvent is a very sizable expense.
While the more recent electrostatic powder-coating technique employs no solvent, such a technique involves the use of costly coating material. This method operates on the principle of transporting a finely divided dry powder and for this purpose, bulk resin must be crushed to a fine, rather uniform particle size and mixed with pigments, fillers, hardeners and the like by sophisticated and rather expensive crushing and mixing equipment. Such equipment includes ball mills, hammer mills, kibblers, extruders, kneaders, and other compounding equipment; filters, sieves, conveyors and the like, all of which are employed to process the coating material into a dry powder form suitable for transportation to the atomizing equipment. But still the technical material problems remain in the electrostatic powder-coating technique because it is difficult to provide satisfactory dry powders which have long shelf-lives for handling and spraying, etc., and these problems diminish the solventless appeal of the powder-coating techniques.
An important part of this brief overview of background for this invention is the sophistication in coating materials that has occurred. The search for a high quality polymeric coating material which can be applied without air pollution has been extensive. However, for instance in the spray application of molten polymers or concentrated polymeric solutions, techniques have not advanced to any significant extent because of the formulators' lack of understanding of atomizing mechanisms and by a similar lack of understanding by spray equipment designers as to the nature of high polymeric liquids. There have been many studies undertaken which relate to theoretical energies required, and the relationship of viscosity, surface tension, temperature, etc., of the liquid coatings. However, for use with high polymers and their concentrated solutions, the viscosity measurements are relatively meaningless and often misleading as comparative indicators of the relative ease or difficulty in atomizing two different polymeric liquids. Rather, polymeric liquids are vastly different from Newtonian liquids. They are somewhat elastic, resist deformation by rapidly applied forces and exhibit varying degrees of spring-back or recoil. Presently there are no practical instrumentations capable of evaluating these values of polymeric liquids so that the forecast of their atomizability or energy required to convey them to a substrate can be achieved. At each stage of the process for atomizing and conveying a polymeric liquid to a surface, the liquid resists high speed deformation. Therefore, it may be understood why solvent additions have been employed because they have the effect of separating the polymeric molecules and facilitating their relative movement to make the solution easier to deform at high speeds and thus easier to atomize. However, even after considerable effort over many years has been expended to prepare high solids coating compositions containing above 50% by volume of polymeric and pigmentary solids, still little success has been achieved, and from 15 to 40% by volume of liquid solvent components is necessary in spite of these efforts.
In summary, the coating and finishing industry is still seeking ways and means to apply polymeric coating compositions without emission of polluting solvents and vapors, and with minimum expenditure of energy per unit of coating material applied. There is a substantial need for efficient and economical processes which are devoid of the problems associated with known techniques for coating surfaces.